Turbine rotor blades and blade root receiving slots, which are formed on the rotor and in which the turbine rotor blades are held, are subjected to very high mechanical and also chemical loads during operation of the turbine. This causes, inter alia, crack formation within the blade root receiving slots, which can greatly reduce the service life of the turbine rotor. Within the context of maintenance and repair work, crack findings are recorded accordingly, and the remaining service life of the rotor is determined by calculation. If this is inadequate, suitable countermeasures must be taken. These can involve grinding out the identified cracks. Alternatively or in addition, it is however also possible to effect a repair by means of the rotor steeples defined by the blade root receiving slots. To that end, individual worn rotor steeples can be removed and can be replaced by welding on corresponding replacement rotor steeples. It is in certain cases also possible to modify the rotor geometry and/or the blade root receiving slot geometry.
When recording crack findings, the cracks are located and their depth is determined, since the crack depth has a considerable influence on the subsequent calculations of the service life of the rotor and crack growth. Non-destructive methods, such as magnetic particle inspection or eddy current testing (ECT) are usually used for detecting cracks that are present. In order to determine the crack depth, the cracks are machined mechanically using a milling machine until the base of the crack is reached. This involves alternating between material removal and checking, using non-destructive crack testing methods, whether the base of the crack has been reached. This can also involve the use of magnetic particle inspection or eddy current testing.
However, an important drawback of this approach is that the milling is performed manually. Accordingly, the milling does not create defined and/or axially constant machining contours, making it difficult to reliably calculate the remaining service life and/or the expected crack growth.